1. Field of the Invention
This invention relates to a lead frame used for a semiconductor device.
2. Description of the Related Art
Conventionally, a semiconductor device is known which comprises: a lead frame having a die pad area and a plurality of leads around the die pad area; a semiconductor chip mounted on said die pad area and having a plurality of connecting terminals; metal wires for connecting tips of the respective leads to the terminals of said semiconductor chip; and a mold resin material for hermetically sealing at least a region including the semiconductor chip and the tips of the leads around the semiconductor chip.
In such a semiconductor device, to stably and easily connect the semiconductor chip to the leads by wires, the wire bonding area of each lead of the lead frame is electroplated with gold or silver. This is both expensive and laborious. Recently, however, it has been proposed that, to improve the production rate, the wire bonding area of the lead frame is not electroplated with gold or silver, but the wire is directly connected by a thermo-sonic method to the wire bonding area of the lead frame. In this case, a copper material is exposed on the wire bonding area of the lead frame which is made of copper or copper alloy electroplated with copper, or not electroplated.
As mentioned above, in a lead frame having a wire bonding area to which a wire is directly connected, the wire bonding areas of the lead frame are not electroplated with gold or silver, but a copper material is directly exposed to the outside. Therefore, the wire bonding area of the lead frame may be easily oxidized by oxygen in the atmosphere. Thus, a copper oxidized film may form on the surface of the wire bonding area while the lead frame is transported or a semiconductor chip is fixed on the die pad area in a heated condition. In such a lead frame, therefore, the copper oxidized film makes it impossible to directly connect the wire to the wire bonding area by a thermo-sonic method.
To prevent the copper oxidized film from being formed on the wire bonding area on which a copper material is exposed, the lead frame is packed in a vacuum condition while the lead frame is transported or put in a reduction environment filled with an inert gas, during wire bonding process by thermo-sonic method. These proposals have drawbacks such as the need for a large amount of laborious work, special installations, and expensive inert gases.